Download Photocatalytic degradation of phenol in water solutions using ZnO

Photocatalytic degradation of phenol in water solutions using ZnO
nanoparticles immobilized on glass
Sedigheh Saeedi1, Hatam Godini2, Mohammad Almasian3, Ghodratollah Shams-Khorramabadi
-Khorramabadi1,
1
1
1
Bahram Kamarehie , Parvin Mostafaie , Fatemeh Taheri
1 Department of Environmental Health Engineering, School of Health, Lorestan University of Medical Sciences,
Khorramabad, Iran
2 Department of Environmental Health Engineering, School of Health, Alborz University of Medical Sciences,
Karaj, Iran
3 Department of English Language, School of Medicine, Lorestan University of Medical Sciences, Khorramabad, Iran
Original Article
Abstract
Phenol and its derivatives are pollutant compounds that are present in the wastewater of many industries. The
objective of this study was to investigate the photocatalytic degradation of phenol in water containing various
concentrations of sodium chloride. A laboratory study was conducted to evaluate the performance of UV/ZnO
process on the efficiency of phenol removal from saline water with ZnO nanoparticles fixed on glass using UVC
radiation. The effects of pH, contact time, sodium chloride concentrations, and the initial concentration of phenol
on the photocatalytic removal of phenol were studied. The photocatalytic degradation of phenol showed suitable
efficiency under the absence of sodium chloride (100% phenol removal at a concentration of 5 mg/l and during
120 minutes). However, the removal efficiency decreased in the presence of a concentration of 30 g/l of sodium
chloride (92.4%). Additionally, phenol photocatalytic degradation efficiency decreased as a result of an increase in
the initial concentration of phenol and the efficiency increased as a result of a decrease in pH (pH = 3). The results
obtained from this study indicated that ZnO nanoparticles or ultraviolet rays alone cannot remove phenol fully and
have a much lower efficiency in comparison with the photocatalytic degradation of phenol. Thus, the photocatalytic
degradation process (UV/ZnO) is an effective method of removing phenol from saline water solutions.
KEYWORDS: Degradation, Phenol, Water Pollution, Nanoparticles
Date of submission: 17 Apr 2015, Date of acceptance: 22 Jun 2015
Citation: Saeedi S, Godini H, Almasian M, Shams-Khorramabadi Gh, Kamarehie B, Mostafaie P, et al.
photocatalytic degradation of phenol in water solutions using ZnO nanoparticles immobilized on glass. J
Adv Environ Health Res 2015; 3(3): 204-13.
Introduction1
Most industrial wastewaters have high
concentrations of salt and dangerous organic
pollutants. Different industries including
petroleum refineries, petrochemical plants,
olive oil production, pesticide production, and
textile, leather, food, and agricultural
Corresponding Author:
Hatam Godini
Email: [email protected]
204
industries produce phenolic wastewaters.1,2
Aromatic pollutants, especially phenol and its
derivatives, are frequently found in the
environment due to their widespread use.3
Phenol is a cyclic hydrocarbon which is
colorless in pure form and is highly soluble in
water, and therefore, it is likely to be present in
water sources. Due to their distinct properties,
including toxicity, effect on the flavor and odor
of water, and adverse effects on human health
and living creatures, phenolic compounds are
J Adv Environ Health Res, Vol. 3, No. 3, Summer 2015
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Saeedi et al.
Photocatalytic Degradation of Phenol
classified as priority pollutants by the US
Environmental Protection Agency (USEPA).4,5
Phenol is quickly absorbed through the skin
and can lead to skin and eye burns upon
contact. Excessive contact with phenol can lead
to coma, seizures, cyanosis, and death.6 Due to
the high toxicity of phenolic compounds, EPA
has recommended the permissible level of 1
mg/l in effluents to be released into the
environment.7 Additionally, the World Health
Organization (WHO) has considered a
concentration of 0.001 mg/l of phenolic
compounds in drinking water as the maximum
permissible concentration.8 Biological treatment,
extraction from solution, reverse osmosis,
chemical oxidation, and electrochemical methods
are the most important methods of removal of
phenol and phenolic compounds from saline
wastewaters. High costs, low efficiency, and the
generation of dangerous and toxic byproducts
are among the factors which limit the use of
some of these methods of removal.9
Recently, another technique called advanced
oxidation has been used for the removal of
contaminants which involves the generation of
very strong oxidation agents such as hydroxyl
radicals.10 The photocatalytic degradation
process is considered as a novel treatment
method, because the catalyst can be easily
activated under normal temperature and
pressure.11 This method is a possible technique
for the removal of organic and non-organic
pollutants from the air and water.12 In
photocatalytic degradation, pollutants are
degraded under the radiation of UV rays and
in the presence of metal oxide particles like
ZnO and TiO2.13 One of the advantages of ZnO
over TiO2 is that ZnO can absorb the higher
ultraviolet (UV) spectrum, and this is due to
the absorption threshold of this compound
which is at the 425 nm wavelength.14
Additionally, ZnO is less toxic, is cheaper, and
in some cases it is more active than TiO2.15
Many researchers have studied the removal
of various refractory organic pollutants using
photocatalytic processes. However, few articles
have investigated the efficiency of this
technique in saline environments. In a study on
the photocatalytic degradation of phenol via
UV/TiO2 in solutions with various salt
concentrations, Azevedo et al. were able to
obtain a removal efficiency of 90% for a phenol
concentration of 50 mg/l and a pH of 7.16 In a
study on the photocatalytic degradation
(UV/TiO2) of humic acid in saline water, AlRasheed and Cardin reported that the rate of
photocatalytic degradation of humic acid was
slower in water with high salinity (46 g/l) in
comparison with water with low salinity
(2.7 mg/l).17 Kashif and Ouyang studied the
inhibitory effects of some anions on the
photocatalytic degradation of phenol.18 The
results of their study showed that chloride ions
have a negative effect on the process. In an
investigation of the effect of various
concentrations of sodium chloride, sulfate, and
nitrate on the photocatalytic degradation of
2-phenylphenol, Khodja et al. concluded that
these anions have highly negative effects on
the degradation of this pollutant.19
Due to the higher capability of ZnO in the
absorption of the UV spectrum in comparison
with TiO2, ZnO nanoparticles were used in the
present study. Additionally, given the
difficulty of separating the nanoparticles
during their use in the form of a suspension
and the need for special equipment for their
filtration from the solution, photocatalysts
fixed on glass, which are completely neutral
substances, were used in the present study.
The objective of this research was to study
the performance of the photocatalytic UV/ZnO
process in the removal of phenol in four saline
concentrations (0, 10, 20, and 30 g/l), taking
into account the effects of other parameters
such as solution pH, initial concentration of
phenol, and contact time.
Materials and Methods
This research was discontinuously conducted
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Saeedi et al.
Photocatalytic Degradation of Phenol
on a laboratory scale in a plexiglass reactor
with a capacity of 1 l. This reactor consists of
two segments. The first segment functioned as
a media on which ZnO fixed on glass and the
solution
containing
the
pollutant
(a
combination of various concentrations of
phenol and NaCl) were placed. The second
segment functioned as the cover of the reactor
which
allowed
5 lamps to be installed on its walls. UVc lamps
and their electrical transformers were placed in
a wooden case in the cover segment with a
distance of 1 cm from the reactor media in
order to prevent the dissemination of the rays
into the environment. This reactor has an entry
orifice and an exit orifice which are situated on
its two sides and the entry orifice is lower than
the exit orifice. The entry and exit orifices are
connected by a special tube to a peristaltic
pump with a maximum rotational speed of
120 rpm so that complete mixing occurs inside
the reactor. Figure 1 shows a diagram and
figure of this reactor.
Figure 1. A figure and diagram
noncontinuous rotational reactor
of
the
1. The reactor cover and the lamps attached to it; 2. The
glass bed containing fixed zinc oxide nanoparticles;
3. Entry orifice; 4. Exit orifice; and 5. Peristaltic pump
206
To fix ZnO nanoparticles, the thermal fixing
method was used.20 First, sandblasted glass
surfaces suited to the size of the reactor were
prepared. Then, the pieces of glass were placed
in a 50% solution of sodium hydroxide for
24 hours. After removal from the solution, they
were washed with distilled and tap water. In
the next stage, a 3% suspension of ZnO
nanoparticles was prepared and mixed using a
shaker (Behdad Co., Iran) for half an hour.
Moreover, to distribute the ZnO nanoparticles
homogeneously in the solution, it was placed
in an ultrasonic bath (SONER 203 H, Rocker
Scientific Co., Taiwan) machine with a
frequency of 50 KHz under the influence of
ultrasonic waves. Then, 5 cc of this suspension
in which the nanoparticles were completely
separated was spread evenly on each piece of
glass, each of which had been dried and
weighed. The pieces of glass were placed in a
hot air oven (DSL 60, Texcare Instrument Co.,
India) at a temperature of 40 to 50 ºC for 6
hours so that they would dry slowly. Then, the
temperature of the hot air oven was raised to
110 ºC for one hour, and in the final stage, the
pieces of glass were placed in an electric
furnace (FT 1200, Meta Therm Furnace Pvt.
Ltd., India) at a temperature of 450 ºC for 1
hour.20 Ultimately, to set aside the pieces of
glass on which nanoparticles were poorly fixed
and were not stable enough, they were rinsed
with distilled water along the reactor flow.
In the present study, phenol with a purity of
99% was purchased from Merck & Co.,
Germany. ZnO nanoparticles with laboratory
level purity were purchased from the British
DPH Company. X-ray diffraction (XRD) tests
were performed using a Philips PNA analytical
diffractometer
and
scanning
electron
microscope (SEM) equipped with an energydispersive X-ray spectroscopy (EDX) system
using Seron AIS2300C on the nanoparticles
powder and on the nanoparticles fixed on the
glass. All chemicals used in this study
including chloridric acid and sodium
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Saeedi et al.
Photocatalytic Degradation of Phenol
hydroxide (used to adjust the pH), sodium
chloride (for the preparation of the saline
solution), and the reagents used in the
colorimetry of phenol including potassium
ferricyanide,
phosphate
buffer,
and
ammonium hydroxide were bought from
Merck & Co. Other equipment used in this
study included 5 ultraviolet 6 watt lamps
(Philips), UV meter (UVc-254, Luetern,
Germany), pH meter (Hana-211, Germany),
UV/vis spectrophotometer (2100 Unico, US),
and peristaltic pump (Heidolph pump drive
5001, Germany).
First, a stock phenol solution (1 g phenol in
1000 cc distilled water) was prepared. Next, in
order to produce the calibration curve, various
concentrations were prepared from this
solution. Additionally, in later stages of the
experiment, the same solution was used to
prepare the desired samples of phenol with the
concentrations required for the experiments via
dilution with distilled water. To determine the
concentration of phenol in unknown and
standard samples, the direct colorimetry
method was used with 4-aminoantpyrine as
the reagent at a wave length of
500 nanometers, using the 5530D colorimetry
method as presented in the standard methods
for the examination of water and wastewater.21
To make the concentration of phenol in the
studied samples similar to the amount found in
industrial wastewaters, the phenol solutions
were prepared in five concentrations of 5,
10, 20, 40, and 80 mg/l. Moreover, four
concentrations
of
sodium
chloride
(0, 10, 20, and 30 g/l) were mixed with various
concentrations of phenol to obtain saline
wastewater. To determine the optimum pH in
photocatalytic degradation, the experiments
were carried out in five amounts of pH
(i.e., 3, 5, 7, 9, and 11). To adjust the pH, 0.1
normal hydrochloridric acid and sodium
hydroxide solutions were used. During each
process and at time intervals of 30, 60, 90, 120,
150, and 180 minutes, samples were taken from
the solution content of the reactor. It is also
necessary to state that after each stage of the
experiments, the used glass surfaces were
heated at a temperature of 450 ºC in the furnace
to reactivate them and make sure that the
performance of the fixed nanoparticles was
acceptable. To compare the results, the samples
were exposed to UV rays alone, ZnO
nanoparticles alone, or both UV rays and ZnO
nanoparticles in separate stages. It should also
be mentioned that all experiments were carried
out
at
laboratory
temperatures.
All
experiments were conducted in triplicate.
To analyze the data, repeated measures
model and longitudinal models (generalized
linear models) were used in SPSS software
(version 21, SPSS Inc., Chicago, IL, USA).
Results and Discussion
The X-ray diffraction result of ZnO
nanoparticles used in the present research is
presented in Figure 2. In figures 3 and 4, the
nanoparticles fixed on glass surfaces, and the
physical properties, size, degree of purity, and
the composition of the particles are illustrated.
Figure 2. X-ray diffraction results of ZnO
nanoparticles
The results obtained from XRD patterns
indicate that the ZnO nanoparticles used had a
hexagonal structure and were of a high degree
of purity. Additionally, sharp peaks indicate
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Saeedi et al.
Photocatalytic Degradation of Phenol
good crystallization of the ZnO nanoparticles.
The results obtained from SEM micrographs
also showed that the size of the ZnO
nanoparticles used in this study was less than
100 nm. Furthermore, after fixing them on
glass, porosity rates remained at a desirable
level and the dimensions of the nanoparticles
was still at the nano level. In addition,
considering the EDX spectrum, only ZnO
nanoparticles could be observed on the glass
surfaces and there were no other impurities.
The results of the adsorption of phenol on
the surface of ZnO nanoparticles are presented
in figure 5. In this figure, it can be observed
that the adsorption of phenol on the surface of
the nanoparticles is negligible if compared
with the photocatalytic degradation of phenol
in the UV/ZnO process. A survey of the effect
of the separate contact of the synthetic
wastewater containing the pollutant with ZnO
nanoparticles, as shown in figure 5, showed
that phenol removal rates were negligible after
180 minutes of contact time. Gaya et al., in a
study on the effect of ZnO alone on the
removal
of
4-chlorophenol
with
a
concentration of 50 mg/l, observed no
considerable change in the concentration of the
pollutant after 300 minutes.22
Only ZnO
Figure 3. Scanning electron microscope (SEM)
micrograph of ZnO nanoparticles fixed on glass
with a magnitude of 30000
Removal efficiency (%)
100
UV+ZnO
80
60
40
20
0
0
30
60
90
120
150
180
Contact time (min)
Figure 5. Comparison of the efficiency of ZnO
nanoparticles alone and the UV/ZnO process in
the removal of phenol (pH = 3, Phenol = 20 mg/l,
and NaCl = 10 g/l)
Energy (KeV)
Figure 4. The results of X-ray spectroscopy
(EDX) analysis of ZnO nanoparticles fixed on
glass
208
The results of the effect of UV radiation
alone on the removal of phenol are presented
in figure 6 (A and B). Results show that
although the use of UV rays alone can be
effective in the removal of lower concentrations
of phenol, in higher concentrations, the
removal efficiency is reduced considerably.
Moreover, it is less effective in comparison
with the photocatalytic process of UV/ZnO.
Phenol removal efficiency was higher at the
lowest studied concentration (5 mg/l), but, at
higher concentrations (80 mg/l), the removal
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Saeedi et al.
Photocatalytic Degradation of Phenol
Only UV
110
90
A
ZnO+UV
B
ZnO+UV
80
100
70
Removal efficiency (%)
90
Removal efficiency (%)
Only UV
80
70
60
50
40
30
60
50
40
30
20
20
10
10
0
0
0
30
60
90
120
Contact time (min)
150
180
0
30
60
90
120
Contact time (min)
150
180
Figure 6. Comparison of the use of UV rays alone and the UV/ZnO process in the removal of phenol
2
(pH = 3, NaCl = 10 g/l, radiation intensity = 3950 µw/cm , and (a) Cphenol = 5 mg/l, (b) Cphenol = 80 mg/l)
known as another factor affecting the pH. This
phenomenon can be explained by the surface
properties of ZnO.
100
Removal efficiency (%)
efficiency decreased because not enough
hydroxyl radicals (the main factor for pollutant
degradation) were generated.23 Given the
above-mentioned issues, it can be said that the
use of these processes alone is not highly
effective in the removal of phenol. Thus, it is
necessary to modify the process and use the
nano-photocatalytic process UV/ZnO in order
to achieve higher removal efficiencies.
The results obtained from the study of the
effects of pH variation in the UV/ZnO process
with 5 amounts of pH are presented in figure 7.
As seen in figure 7, the phenol removal
efficiency was higher under acidic conditions
than under basic conditions; the highest
removal
efficiency
for
the
studied
concentrations of phenol was at pH of 3
(90.2%) and the lowest removal efficiency was
at pH of 11 (71.3%).
Two reasons can be mentioned for this
phenomenon. The first reason may be the fact
that higher numbers of H+ ions are present in
the acidic environment which leads to the
formation of H˚ radicals which also form HO2˚
radicals with the oxygen present in the
solution, which finally convert to OH˚
radicals.23 Electrostatic reactions between
phenol and the surface of the catalyst are
3
5
7
9
11
80
60
pH
40
20
0
0
30
60
90
120
Contact time (min)
150
180
Figure 7. The effect of pH on the removal of
phenol in the UV/ZnO process (the initial
concentration of phenol = 40 mg/l, NaCl = 10 g/l,
2
and radiation intensiy = 3950 µw/cm )
In this study, the pHzpc (point of zero
charge) of the ZnO nanoparticles was about
8.60. At pH levels higher than this, the surface
charge of the ZnO nanoparticles was negative,
while at pH levels lower than this, the particles
had a positive surface charge.24 Many ionic
compounds like phenols have negative charges
(anionic compounds) which can be easily
attracted to the surface of ZnO particles at pH
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Saeedi et al.
Photocatalytic Degradation of Phenol
levels less than pHzpc. However, at pH levels
higher than pHzpc, ZnO nanoparticles repel
each other because they have negative
charges.25 The results obtained from the
statistical analysis also showed that the
difference in the mean phenol removal rates
was significant at various pH levels (P< 0.001).
In their investigation of the effect of pH
(2.5 to 12.5) on the removal of phenol and
benzoic acid using the UV/ZnO photocatalytic
process, Mrowetz and Selli reported that the
highest removal efficiency was obtained at the
pH of 2.5.26
The results of the phenol removal efficiency
at various concentrations of sodium chloride
(0, 10, 20, 30 g/l) are shown in figure 8. As can
be seen in figure 8, an increase in the
concentration of sodium chloride (NaCl) leads
to a reduction in the removal efficiency. The
highest removal efficiency (92.4%) was
obtained in the absence of sodium chloride,
and the lowest removal efficiency (81.12%) was
obtained for the 30 g/l concentration of sodium
chloride at the same contact time (180 minutes).
100
0
10
20
30
90
70
60
120
5
50
CNaCl(g/l)
40
30
20
10
0
0
30
60
90
120
150
10
20
40
80
100
Removal efficiency (%)
Removal eficiency (%)
80
Regarding the negative effect of the various
concentrations of NaCl on the photocatalytic
degradation of phenol and the reduction of
removal efficiency with the increase in saline
concentration (NaCl) in the studied solution
(Figure 8), it can be said that the reduction in the
efficiency of the process can be attributed to the
presence of chloride ions, because chloride ions
are able to adsorb hydroxyl radicals.27
Additionally, it seems that the inhibitory effect of
sodium chloride is due to the joining of the
chloride ions to electrons and optical cavities. The
chloride ions, which have been adsorbed to these
cavities and have been oxidized, are converted to
chlorine atoms, and then, again are reduced as
chloride ions by the electrons.16 The results
obtained from the repeated measures model
statistical analysis also indicated a significant
difference in the mean phenol removal efficiency
at various NaCl concentrations (P = 0.004).
Moreover, the results suggest that sodium
chloride has a negative effect on the oxidation of
phenol. Papadam et al.28 and LAmour et al.29
obtained similar results in their studies.
The study of the effect of initial concentration
of phenol on photocatalytic degradation (for the
optimum pH obtained in the previous stage) has
shown that as the initial concentration of phenol
increased, the photocatalytic degradation rates
decrease (Figure 9).
180
Contact time (min)
80
60
CPhenol
(mg/l)
40
20
0
0
30
60
90
120
150
180
Contact time (min)
Figure 8. The effect of various concentrations of
sodium chloride on the removal of phenol in the
UV/ZnO process (pH = 3, phenol = 40 mg/l, and
2
radiation intensity = 3950 µw/cm )
210
Figure 9. The effect of the initial concentration
of phenol and contact time on the removal of
phenol in the UV/ZnO process (pH = 3, and
2
radiation intensity = 3950 µw/cm )
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Saeedi et al.
Photocatalytic Degradation of Phenol
Therefore, removal efficiency has an inverse
relationship with the initial concentration of
phenol. Additionally, the time needed to achieve
a desirable removal efficiency increased as the
initial concentration of phenol was raised. After
120 minutes and at an acidic pH, the highest
removal efficiency (100%) was achieved at the
lowest concentration (5 mg/l), and the lowest
removal rate at a concentration of 80 mg/l and a
contact time of 180 minutes was 76.5%.
In investigating the effect of the initial
concentration of phenol and contact time on
photocatalytic degradation, as can be seen in
figure 9, as the initial concentration of phenol
increased, the photocatalytic degradation rates
decreased. In explaining this issue, it can be
stated that, in the photocatalytic process, as a
result of the increase in the amount of influent
pollutant, the likelihood of collision of the
pollutant particles with the oxidizing agent
(i.e., hydroxyl) is reduced, which leads to a
reduction in the efficiency of the removal
system at higher concentrations. Another
reason for this issue is the generation of
byproducts which are more reactive than
phenol and which react with the present
radicals or are adsorbed onto the surface of the
photocatalyst; therefore, these byproducts
compete with phenol in adsorption onto active
sites on the ZnO surface and in reacting with
hydroxyl radicals.26,30 The results obtained
from statistical analysis have demonstrated the
significant difference between mean phenol
removal
efficiencies
at
its
various
concentrations (P < 0.001). Pardeshi and Patil31
and Chiou et al.32 have also reached similar
results in their studies. Moreover, Zamankhan
et al.33, in a study on the removal of phenol
using the photocatalytic method and ZnO
nanoparticles, concluded that an increase in
contact time leads to an increase in the removal
efficiency of phenol.
Kinetics results of phenol removal through
the photocatalytic process are shown in figure 10.
Two models, the pseudo-first-order and the
pseudo-second-order, were used to determine
a possible mechanism involved in the
degradation. The R2 for pseudo-first-order
kinetics (R2 = 0.93) was higher than the pseudosecond-order (R2 = 0.75).
The removal of phenol in the UV/ZnO
process fit the pseudo-first-order reaction
patterns well (R2 = 0.93). The photocatalytic
oxidation kinetics of many organic compounds
have often been modeled with the Langmuir–
Hinshelwood equation, which also covers the
adsorption properties of the substrate on the
photocatalyst surface.14 The results of this
study are in agreement with that of the studies
by Behnajady et al.14 and Alalm and Tawfik 34
4
3.5
A
y = 0.0314x - 0.1983
3
3.5
B
y = 0.0388x - 0.5301
3
2.5
2.5
R² = 0.75
2
2
1/Ct
Ln C0/C
R² = 0.9
1.5
1.5
1
1
0.5
0
0.5
-0.5
0
0
20
40
Time (min)
60
80
100
0
-1
20
40
60
80
100
Time (min)
Figure 10. Phenol removal kinetics in the UV/ZnO process: (A) the pseudo-first-order and (B) the pseudosecond-order (pH = 3, retention time = 90 minutes, and phenol concentration = 5 mg/l)
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Photocatalytic Degradation of Phenol
Conclusion
In this study, the removal of phenol from
saline wastewaters using the UV/ZnO
photocatalytic process was investigated. In
general, the results obtained from this study
show that the use of the UV/ZnO process even
in wastewaters containing anions like chloride,
which is a scavenger of hydroxyl radicals and
has an inhibitory effect on the process, was an
effective method in removing phenol.
However, at higher concentrations of sodium
chloride (30 g/l), the time needed for achieving
the desirable treatment levels increased.
Additionally, ZnO nanoparticles alone or UV
rays alone have a lower efficiency in the
removal of phenol in comparison with the
UV/ZnO photocatalytic process.
Conflict of Interests
Authors have no conflict of interests.
Acknowledgements
We would like to extend our gratitude to the
experts and people in charge of the research
laboratories of the School of Health and the
Razi Herbal Medicines Research Center of the
Lorestan University of Medical Sciences, Iran.
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